Currently, my favorite is stable diffusion. I find it to be the most flexible compared to the other ones.
There's thousands of different models online, and you can run it locally for free (if you have a good enough PC).
DALLE2 and Midjourney are said by many to produce better image, but they're less customizable and have really heavy content filters that remove completely innocent results.
What's your favorite AI image generator and why?
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If anyone developing an app with stable diffusion is interested, I'm currently developing an AI API for stable diffusion you can easily incorporate into your app. We're almost done the backend stuff, and you can see more progress following me on twitter
@TheRealEtch